Infrared-cut filter and imaging optical system

Abstract

An infrared-cut filter ( 1 a ) includes a near-infrared reflecting film ( 20 ) and an absorbing film ( 30 ). The near-infrared reflecting film ( 20 ) and the absorbing film ( 30 ) have the following features (A) to (E); (A) 700 nm≤λ H R (0°, 70%)<λ H R (0°, 20%)≤770 nm; (B) 650 nm≤λ H R (40°, 70%)<λ H R (40°, 20%)≤720 nm; (C) λ H A (40°, 20%)<λ H R (40°, 20%); (D) the spectral transmittance of light incident on the absorbing film ( 30 ) is 15% or less at λ H R (0°, 20%) and λ H R (40°, 20%); and (E) an average of the spectral transmittance of light incident on the near-infrared reflecting film ( 20 ) and an average of the spectral transmittance of light incident on the absorbing film ( 30 ) are 75% or more in the wavelength range of 450 to 600 nm.

The invention claimed is: 
     
       1. An infrared-cut filter comprising:
 a near-infrared reflecting film; and 
 an absorbing film lying parallel to the near-infrared reflecting film, wherein 
 the near-infrared reflecting film and the absorbing film have the following features (A) to (E): 
 (A) when a wavelength which is in the wavelength range of 600 to 800 nm and at which spectral transmittance of light perpendicularly incident on the near-infrared reflecting film is 70% is defined as a wavelength λ H   R  (0°, 70%) and a wavelength which is in the wavelength range of 600 to 800 nm and at which the spectral transmittance of light perpendicularly incident on the near-infrared reflecting film is 20% is defined as a wavelength λ H   R  (0°, 20%), the spectral transmittance of light perpendicularly incident on the near-infrared reflecting film monotonically decreases in the range from the wavelength λ H   R  (0°, 70%) to the wavelength λ H    R  (0°, 20%) in such a manner that the wavelength λ H   R  (0°, 70%) is 700 nm or longer and the wavelength λ H   R  (0°, 20%) is 770 nm or shorter and is longer than the wavelength λ H   R  (0°, 70%); 
 (B) when a wavelength which is in the wavelength range of 600 to 800 nm and at which spectral transmittance of light incident on the near-infrared reflecting film at an incident angle of 40° is 70% is defined as a wavelength λ H   R  (40°, 70%) and a wavelength which is in the wavelength range of 600 to 800 nm and at which the spectral transmittance of light incident on the near-infrared reflecting film at an incident angle of 40° is 20% is defined as a wavelength λ H   R  (40°, 20%), the spectral transmittance of light incident on the near-infrared reflecting film at an incident angle of 40° monotonically decreases in the range from the wavelength λ H   R  (40°, 70%) to the wavelength λ H   R  (40°, 20%) in such a manner that the wavelength λ H   R  (40°, 70%) is 650 nm or longer and the wavelength λ H   R  (40°, 20%) is 720 nm or shorter and is longer than the wavelength λ H   R  (40°, 70%); 
 (C) spectral transmittance of light incident on the absorbing film at an incident angle of 40° is 20% at a wavelength λ H   A  (40°, 20%) which is in the wavelength range of 600 to 800 nm and which is shorter than the wavelength λ H   R  (40°, 20%); 
 (D) spectral transmittance of light perpendicularly incident on the absorbing film is 15% or less at the wavelength λ H   R  (0°, 20%), and the spectral transmittance of light incident on the absorbing film at an incident angle of 40° is 15% or less at the wavelength λ H   R  (40°, 20%); and 
 (E) an average of the spectral transmittance of light perpendicularly incident on the near-infrared reflecting film and an average of the spectral transmittance of light incident on the near-infrared reflecting film at an incident angle of 40° are 75% or more in the wavelength range of 450 to 600 nm, and an average of the spectral transmittance of light perpendicularly incident on the absorbing film and an average of the spectral transmittance of light incident on the absorbing film at an incident angle of 40° are 75% or more in the wavelength range of 450 to 600 nm. 
 
     
     
       2. The infrared-cut filter according to  claim 1 , wherein the near-infrared reflecting film is formed by alternately arranging two or more materials with different refractive indices in layers, and the absorbing film contains a substance having an absorption peak in the range from the wavelength λ H   R  (40°, 70%) as defined for the feature (B) to the wavelength λ H   R  (0°, 20%) as defined for the feature (A). 
     
     
       3. The infrared-cut filter according to  claim 1  or  2 , wherein an absolute value of a difference between a wavelength λ H  (0°, 50%) which is in the wavelength range of 600 to 700 nm and at which spectral transmittance of light perpendicularly incident on the infrared-cut filter is 50% and a wavelength λ H  (40°, 50%) which is in the wavelength range of 600 to 700 nm and at which spectral transmittance of light incident on the infrared-cut filter at an incident angle of 40° is 50% is 10 nm or less. 
     
     
       4. The infrared-cut filter according to  claim 1 , wherein the near-infrared reflecting film and the absorbing film further have the following features (F) to (I):
 (F) in the wavelength range of 350 to 450 nm, the spectral transmittance of light perpendicularly incident on the near-infrared reflecting film monotonically increases in the range from a wavelength λ L   R  (0°, 20%) at which the spectral transmittance is 20% to a wavelength λ L   R  (0°, 70%) at which the spectral transmittance is 70% in such a manner that the wavelength λ L   R  (0°, 20%) is 390 nm or longer and is shorter than the wavelength λ L   R  (0°, 70%); 
 (G) in the wavelength range of 350 to 450 nm, the spectral transmittance of light incident on the near-infrared reflecting film at an incident angle of 40° monotonically increases in the range from a wavelength λ L   R  (40°, 20%) at which the spectral transmittance is 20% to a wavelength λ L   R  (40°, 70%) at which the spectral transmittance is 70% in such a manner that the wavelength λ L   R  (40°, 20%) is 370 nm or longer and is shorter than the wavelength λ L   R  (40°, 70%); 
 (H) in the wavelength range of 350 to 450 nm, the spectral transmittance of light incident on the absorbing film at an incident angle of 40° monotonically increases in the range from a wavelength λ L   A  (40°, 20%) at which the spectral transmittance is 20% to a wavelength λ L   A  (40°, 50%) at which the spectral transmittance is 50% in such a manner that the wavelength λ L   A  (40°, 20%) is 370 nm or longer and is shorter than the wavelength λ L   A  (40°, 50%); and 
 (I) in the wavelength range of 350 to 450 nm, the spectral transmittance of light incident on the absorbing film at an incident angle of 40° is 60% or less at a wavelength λ L   R  (0°, 50%) at which the spectral transmittance of light perpendicularly incident on the near-infrared reflecting film is 50%. 
 
     
     
       5. The infrared-cut filter according to  claim 4 , wherein an absolute value |Δλ L  (50%)| of a difference between a wavelength which is in the wavelength range of 350 to 450 nm and at which spectral transmittance of light perpendicularly incident on the infrared-cut filter is 50% and a wavelength which is in the wavelength range of 350 to 450 nm and at which spectral transmittance of light incident on the infrared-cut filter at an incident angle of 40° is 10 nm or less. 
     
     
       6. The infrared-cut filter according to  claim 1 , wherein the near-infrared reflecting film has the following feature (J):
 (J) the spectral transmittance of light incident on the near-infrared reflecting film at an incident angle of 40° has a spectrum which has a minimum differing by 10 points or more from a baseline in the wavelength range of 400 to 450 nm, in which a half-width for the minimum is 10 nm or more, and in which, when the half-width is defined as Δλ c , a maximum exists in the range from (400−Δλ c /2) to (450−Δλ c /2) nm. 
 
     
     
       7. The infrared-cut filter according to  claim 4 , wherein assuming that the infrared-cut filter is used with a given imaging sensor and that a B/G ratio representing a spectral sensitivity ratio of the imaging sensor is 1 when determined by making light incident on the infrared-cut filter at an incident angle of 0°, the B/G ratio is 0.97 or more and 1.03 or less when determined by making light incident on the infrared-cut filter at an incident angle of 40°. 
     
     
       8. The infrared-cut filter according to  claim 1 , wherein spectral transmittance of light perpendicularly incident on the infrared-cut filter and spectral transmittance of light incident on the infrared-cut filter at an incident angle of 40° do not have a spectrum that overlaps bright-line spectra of illuminant TL84 which appear in the vicinity of 440 nm, in the vicinity of 550 nm, and in the vicinity of 610 nm, that has an extremum differing by 4 or more points from a baseline, and that has a half-width of 15 nm or more. 
     
     
       9. The infrared-cut filter according to  claim 1 , further comprising a transparent dielectric substrate, wherein
 the near-infrared reflecting film and the absorbing film lie parallel to a principal surface of the transparent dielectric substrate. 
 
     
     
       10. An imaging optical system comprising the infrared-cut filter according to  claim 1 .